Isomorphisms and representations of noncommutative function algebras

Abstract

Let $V$ be a noncommutative (NC) variety in a NC operator ball. We study the dual operator algebra $H^\infty(V)$ of bounded NC functions on $V$, and the operator algebra $A(V)$ obtained as the closure of NC polynomials in $H^\infty(V)$. We identify the space of weak-$*$ continuous finite-dimensional representations of $H^\infty(V)$ as the similarity envelope $\widetilde{V}$ of $V$, and equip $\widetilde{V}$ with a metric which turns $\widetilde{V}$ into a complete invariant for $H^\infty(V)$, in the sense that $H^\infty(V)$ is weak-$*$ continuously isomorphic to $H^\infty(W)$ if and only if $\widetilde{V}$ and $\widetilde{W}$ are NC biholomorphic via a bi-Lipschitz map. Similar results are obtained in the completely bounded category as well as for the algebras $A(V)$ when $V$ is assumed homogeneous. In the case of homogeneous varieties $V$ and $W$, we obtain a sharper classification result, by which $H^\infty(V)$ is (completely) boundedly isomorphic to $H^\infty(W)$ if and only if there is a linear map which gives a bi-Lipschitz bijection between $\widetilde{W}$ and $\widetilde{V}$. The reduction to the linear case in the generality we work in depends on showing that the existence of maps implies the existence of origin-fixing maps, which we show using a novel ``two-circle trick" of independent interest. In the important case where the varieties are the full NC operator balls we find a striking rigidity phenomenon: if $A(B_E)$ and $A(B_F)$ are completely boundedly isomorphic then $E$ and $F$ are completely isometric. We also present a variety of examples illustrating the results and their sharpness, showing how several results from the literature fail to generalize to the setting studied here.